In principle, the expression of a protein proceeds along the pathway of transcribing the information in DNA coding for the protein into mRNA, and then polymerizing the amino acids specified by the nucleotide information in the transcribed mRNA to synthesize (translate) the protein. The amino acids are specified in units of three nucleotides (referred to as “codons” hereinafter) in the mRNA nucleotide information. Each codon corresponds to a single amino acid. However, in some cases, two to six codons each correspond to the same amino acid. For example, there are six codons that correspond to the amino acid leucine in human beings: CUG, UUA, UUG, CUU, CUC, and CUA.
Codons are not necessarily uniformly employed in a target cell. Normally, their use is somewhat lopsided. The frequency of use of each type of codon among the multiple codons specifying a single amino acid (the “use frequency” hereinafter) exhibits diversity in biological species through the process of evolution (Nonpatent References 1 to 3). The use frequency of codons correlates positively with the tRNA concentration within the cell. The concentration of tRNA corresponding to high usage codons is high, and the genes constituted by high usage codons exhibit high levels of expression (Nonpatent References 4 to 8). The fact that the concentration of tRNA corresponding to low usage codons is low in cells has come to be understood. In particular, the concentration in cells of the tRNA corresponding to the codon with the lowest use frequency in terms of a genome (referred to as the “rear codon” hereinafter) is the lowest among the cellular concentrations of the tRNA of all codons (Nonpatent References 9 and 10). Since the use frequency of codons varies between biological species, for example, the expression level of a human-derived gene will decrease in Escherichia coli relative to what it is in a human cell (Nonpatent Reference 10).
Thus far, the method of adjusting the level of expression of a protein within the cell of a living organism of the same or different species has been reported as a technique that involves replacing the codons constituting the mRNA translating a protein with codons of different use frequencies (Patent References 1 and 2). Further, a method of changing the properties of a cell by adjusting the expression level of a protein that is expressed by the cell using the methods described in Patent References 1 and 2 has been reported (Patent Reference 3). For example, it is possible to suppress the growth of a cell by the method described in Patent Reference 3.
Patent Reference 1; Japanese Patent Unexamined Publication No. 2001-509388
Patent Reference 2; Japanese Patent Unexamined Publication No. 2006-500927
Patent Reference 3; Japanese Patent Unexamined Publication No. 2006-506986    Nonpatent Reference 1; Sharp, P. M. & Matassi, Curr Opin Genet Dev. 4, 851-860 (1994).    Nonpatent Reference 2; Bulmer, M. Genetics, 149, 897-907 (1991).    Nonpatent Reference 3; Sharp, P. M., Stenico, M., Peden, J. F. & Lloyd, A. T., Biochem, Soc. Trans. 21, 835-841 (1993).    Nonpatent Reference 4; Ikemura, T., J. Mol. Biol. 151, 389-409 (1981).    Nonpatent Reference 5; Ikemura, T., Mol. Biol. Evol. 2, 13-34 (1985).    Nonpatent Reference 6; Sharp, P. M. & Li, W., J. Mol. Evol. 24, 28-38 (1986).    Nonpatent Reference 7; Anderson, S. G. E. & Kurland, C. G., Microbiol. Rev. 54, 198-210 (1990).    Nonpatent Reference 8; Bennetzen, J. L. & Hall, B. D., J. Biol. Chem. 257, 3026-3031 (1982).    Nonpatent Reference 9; Fumiaki, Y., Yoshiki, A., Akira, M., Toshimichi, I. & Syozo, O., Nucleic Acids Res. 19, 6119-6122 (1991).    Nonpatent Reference 10; Makrides, S. C., Microbiol Rev. 60, 512-538 (1996).
The entire contents of Nonpatent References 1 to 10 are hereby incorporated by reference.